benz

Loose Nut Danger

Blog Post created by benz on Sep 23, 2016

Originally posted Jan 27, 2013

 

Lurking on your workbench

Some hazards come from unexpected directions and find ways to bypass safety measures.  One example is mechanical connector damage to high frequency cables and adapters, especially when SMA and precision 3.5mm connectors are used in an environment where 2.4mm and 1.85mm are also used.  As usual, a picture is worth a thousand words:

The picture of the 3.5 mm-to-2.4 mm adapter at the top center explains things best.  Note that the outer shoulder length of the outer conductor is different and that the thread pitches are different as well.  This ensures that the connector nuts won’t fit if the wrong connectors are used.  So far, so good.

The picture of the 3.5 mm-to-2.4 mm adapter at the top center explains things best.  Note that the outer shoulder length of the outer conductor is different and that the thread pitches are different as well.  This ensures that the connector nuts won’t fit if the wrong connectors are used.  So far, so good.

The danger is easy to see if you examine the collets (female center connectors) at both ends of the adapter, shown in the pictures at the upper left and upper right.  Note the differences in the collet thickness and the acceptable center pin size for the two connectors.  If the center pin of the SMA connector at the lower left (intermatable with precision 3.5 mm) is inserted in the collet of the 2.4mm connector at the upper right, the 2.4mm connector will be ruined.  The damage would be the same if the male connector used was 3.5mm.

But of course the outer nut prevents that from happening.  Or does it?

The safety measure implemented through different connector dimensions and thread pitches (as described above) only works if the nut on the male connector is restrained at the end of the cable and not allowed to slide back or away from the end connector.  As you can see from the picture at the lower right that is not the case with this particular bit of semi-rigid coaxial cable and SMA connector.  The other dimensions of the outer connector are compatible and with a little force the SMA connector can fulfill its damaging destiny.

One piece of good fortune for the unlucky engineer is that female 2.4 mm and 1.85 mm connectors are not generally present on instrument front panels.  Male connectors are used instead (details in an upcoming post) and so the damage is usually limited to adapters and cables.  They’re expensive enough on their own, but much less costly than replacing instrument panel connectors and recalibrating.

Connector nuts on 3.5 mm hardware are almost always restrained but the practice is not universal, and is much less common on inexpensive SMA hardware.  And of course the restraint (sometimes a snap ring) can come adrift on almost any connector.

Thus this hazard is present in any environment where 2.4 mm or 1.85 mm connectors are used, including most millimeter-wave and some microwave applications.

You get bonus hazard detection points if you noticed the additional problem with the slightly extended and bent center pin of the SMA connector at the lower left.

 

This high frequency hardware is expensive and somewhat delicate, and damage can hurt performance even when connectors aren’t destroyed outright.  Gear gets even more expensive and delicate as operating frequencies get higher, so be careful out there!

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